1. Field of the Invention
The present invention relates to an electro-magnetic wave shielding structure for preventing undesired leakage of an electro-magnetic wave, particularly, for preventing an electric wave obstruction in the surrounding environment, which is caused by undesired leakage of an electro-magnetic wave generated in an electric apparatus including a communication apparatus, for instance, a car telephone, a portable telephone and the like.
2. Related Art Statement
Recently, cases or housings of electric apparatus of various kinds have a tendency to be formed of plastics which are superior in that they are light weight and have desirable plasticity.
On the other hand, inner circuits and electric parts, which are comprised in electric apparatus, include articles which generate undesired electro-magnetic waves causing electric wave obstructions. Particularly, communication apparatus, information apparatus, electronic apparatus comprising integrated circuits and the like include many articles which deal with high frequency electric signals together with a high possibility for causing the leakage of high frequency electric waves originating electric wave obstructions.
Accordingly, when the aforesaid case of electric apparatus is formed of plastics having a general property such that an electro-magnetic wave passes therethrough, the aforesaid undesired leakage of the electro-magnetic wave is extremely increased and hence the occurrence of electric wave obstruction becomes a very severe problem.
Indeed, when special plastics provided for shielding the electro-magnetic wave by containing metal powders or the like therein are employed as for the plastics forming the aforesaid case, the leakage of electro-magnetic waves can be prevented to a certain extent.
However, such special plastics are expensive in terms of material costs and further the performances thereof, including plasticity, are lowered, so that the advantage of the case made of plastics cannot be sufficiently obtained.
Therefore, a measure involving applying a conductive layer formed by metal plating or painting a conductive material on the plastics case has been conventionally proposed. A thus applied conductive layer has a property wherein the electro-magnetic wave is reflected thereby. As a result, an electro-magnetic wave generated in the case can be prevented from the being undesirably leaked from the case made of plastics according to the reflection attained by the conductive layer applied thereon.
FIG. 1 shows an outline of an electric apparatus comprising a conventional electro-magnetic wave shielding structure. In FIG. 1, an electro-magnetic wave reflecting layer 20, which is formed of a metalplating layer and the like, is applied on an inner face of a case 10 made of plastics, so that an electromagnetic wave generated in an electro-magnetic wave generating source 12 is reflected at inner faces of the reflecting layer 20 as indicated by arrow marks, so as to be prevented from passing through the case 10 and leaking to the environment.
However, even if the electro-magnetic reflecting layer is formed on the inner face of the plastic made case as described above, leakage of electromagnetic waves nevertheless occurs to an unacceptable level so that improvement of this leakage preventing measure has been demanded.
In further detail, in the case formed of plastics, the leakage of electro-magnetic waves is not caused at portions on which the reflecting layers are applied. However, the case of the electric apparatus is provided with a ventilating hole, an internal inspection hole and other holes, so that the electro-magnetic wave leaks through these holes. The electro-magnetic wave in a frequency region in an order of 1 GHz, which has a large possibility of generation inside the electric apparatus and further a large effect of electric wave obstruction, can be readily leaked even from a minute opening and a minute gap which exist in the case. As a result, it has been difficult to cut the leakage of electro-magnetic waves in the high frequency region.
Moreover, when the electro-magnetic wave reflecting layer has been applied on the inner face of the case made of plastics, the electro-magnetic wave generated in the case is moved entirely within the case by being repeatedly reflected on the reflecting layer with the result that it is concentrically leaked through the openings. Furthermore, the conductive electro-magnetic wave reflecting layer is operated as if it were a half wavelength antenna, with the result that the electro-magnetic wave emitted from the local oscillator inside the case is secondarily emitted therefrom. Consequently, the provision of the electro-magnetic wave reflecting layer cannot have been useful as an effective electro-magnetic wave shielding measure.
In other words, as is shown in FIG. 1, when an opening 14 exists in a portion of the case 10, the electro-magnetic wave being repeatedly reflected by the electro-magnetic wave reflecting layer 20 inside the case 10 reaches to the portion on which the opening 14 exists, and hence readily passes through the opening 14 to the outside thereof. The electro-magnetic wave is hardly attenuated so long as it is reflected by the reflecting layer 20, so that the major part of the electromagnetic wave energy generated from an electro-magnetic wave generating source 12 is possibly leaked through the opening 14 to the outside thereof.